Place the onions in a large saucepan and cover with water. Bring to a boil and cook until the onions are partially tender, about 10 to 15 minutes.

Drain and cool; cut the onions in half crosswise. Scoop out the center of each onion half, leaving a 1/2-inch shell. Reserve the centers. If necessary, cut a small piece from the end of each onion shell so the shells will stand upright.

Prepare a shallow baking dish large enough to hold the onion halves in one layer with non-stick pan spray. Place the onions in the prepared dish, hollowed sides up.

Preheat the oven to 350 degrees F.

Chop the reserved centers of the onions. Saute in oil with the garlic in a medium saucepan until tender, about 5 minutes.

Stir in the spinach, lemon juice, and pepper; cook until the liquid evaporates. Remove from the heat; stir in the bread crumbs and cheese

Fill the onion shells with the spinach mixture. Cover with foil and bake about 25 minutes. Serve hot.